US 3787747 A
The periodic magnet structure for a periodically focused beam tube is formed by a single pair of homogeneous slabs of magnetizable material that are permanently magnetized with a pattern of magnetic poles of alternating polarity taken along the direction of the beam path. In a sheet beam tube of preferred geometry, a pair of ceramic slabs are disposed to straddle the sheet beam and the internal planar faces of the ceramic slabs have circuits printed thereon for: electrical connections to all elements, beam forming electrodes, microwave interaction structure, edge focusing electrodes, and the beam collector electrode structure. The permanently magnetized slabs are disposed external to the vacuum envelope, straddling the printed circuit ceramic slabs, for focusing the beam.
Description (OCR text may contain errors)
Scott [451 Jan. 22, 1974 PERIODIC MAGNETICALLY FOCUSED BEAM TUBE  Inventor: Allan W. Scott, Los Altos, Calif.
 Assignee: Varian Associates, Palo Alto, Calif.
 Filed: Aug. 7, 1972  Appl. No.: 278,408
 US. Cl 315/35, 315/36, 315/535, 335/210  Int. Cl. H01j 25/34  Field of Search..... 315/35, 3.6, 5.35; 335/210, 335/302  References Cited UNITED STATES PATENTS 3,705,327 12/1972 Scott 315/3.5
2,812,470 11/1957 Cook et a1. 315/3.5 X
3,670,196 6/1972 Smith 3l5/3.5
2,911,555 11/1959 Sensiper et a1. 315/35 3,504,222 3/1970 Fukushima 315/3.5 X
3,231,780 1/1966 Feinstein 315/3.5 X
3,610,999 10/1971 Falce 315/35 Primary Examiner Rudolph V. Rolinec Assistant Examiner-Saxfield Chatmon, Jr. Attorney, Agent, or Firm-Stanley Z. Cole [5 7 ABSTRACT The periodic magnet structure for a periodically focused beam tube is formed by a single pair of homogeneous slabs of magnetizable material that are permanently magnetized with a pattern of magnetic poles of alternating polarity taken along the direction of the beam path. In a sheet beam tube of preferred geometry, a pair of ceramic slabs are disposed to straddle the sheet beam and the internal planar faces of the ceramic slabs have circuits printed thereon for: electrical connections to all elements, beam forming electrodes, microwave interaction structure, edge focusing electrodes, and the beam collector electrode structure. The permanently magnetized slabs are disposed external to the vacuum envelope, straddling the printed circuit ceramic slabs, for focusing the beam.
19 Claims, 12 Drawing Figures PATENTEB JAN 22 I974 SHEET 1 or 3 B G F FIG 2B PATENIEUJAHZE? m SHEET 3 [IF 3 FIG.8
PERIODIC MAGNETICALLY FOCUSED BEAM TUBE GOVERNMENT CONTRACT The invention herein described was made in the course of or under a contract with the department of the U. S. Army.
DESCRIPTION OF THE PRIOR ART The present invention relates to the field of periodic permanent magnet focused microwave beam tubes such as traveling wave tubes and klystrons.
Heretofore, it has been proposed to print a microwave interaction structure on the inner faces of a pair of mutually opposed and spaced apart ceramic slabs disposed straddling a sheet-shaped electron beam. The beam was electrostatically focused by means of printed circuit electrodes carried on the ceramic slab either on the same face with that of the circuit or on an opposite face. Tubes of this character are disclosed and claimed in U. S. Pat. Nos. 3,549,852 issued Dec. 22, 1970; 3,448,384 issued June 3, 1969; and U. S. Pat. application Ser. No. 149,191, filed June 2, 1971 now US. Pat. No. 3,705,327.
The disadvantages. of utilizing electrostatic focusing elements are that arcing and secondary electron emission exist between the focusing elements which are at different voltages in the beam path, and the full voltage applied to the tube is not available for microwave interaction, since the effective interaction voltage is a value between the focusing voltages.
It is also known from the prior art that tubular or sheet electron beams may be magnetically focused by means of a periodic magnetic focusing structure having a pole pattern which is characterized by poles of alternating polarity taken in the direction of the beam path. In these prior art periodic magnetic focusing structures, two types of geometries have been employed; one of which employs magnetic poles of the same polarity disposed in registration transversely across the beam and the second geometry wherein poles of opposite polarity are disposed in transverse registration across the beam path. The magnetic focusing structure of the first type having poles of the same polarity disposed transversely of the beam path is disclosed in U. S. Pat. No. 3,102,211 (see FIG. 13) issued Aug. 27, 1963. The second type of geometry wherein the poles are of opposite polarity taken transversely of the beam path is disclosed and claimed in U. S. Pat. No. 3,013,173 issued Dec. l2, 1961.
However, in these prior art periodic magnetic focus ing structures, the magnet structure has been relatively complicated because it requires a plurality of permanently magnetized magnets separated by magnetically permeable material or spacers of a different magnetic property. Thus, the resultant structure is relatively complicated requiring a relatively large number of parts which must be assembled around the envelope of the tube to provide a composite periodic permanent magnet beam focusing structure.
SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved periodic magnetically focusedbeam tube.
In one feature of the present invention, the periodic magnetic focusing structure comprises a structure of generally homogeneous permanently magnetizable material magnetized in a pattern of periodic permanent poles of alternating polarity taken in a direction along the beam path, whereby a relatively complicated periodic permanent pole geometry is obtained with an extremely simple magnetic structure. In another feature of the-present invention, a microwave beam tube includes a microwave interaction structure formed on a major face of a dielectric slab facing the electron beam and a permanently magnetized pair of slabs of generally homogeneous magnetic material forms the beam focusing structure disposed external of the vacuum envelope of the tube adjacent the outside wall of the dielectric slab for causing the magnetic fields to permeate the dielectric slab and microwave interaction structure to focus the beam internally of the vacuum envelope of the tube.
In another feature of the present invention, a microwave interaction structure, electrostatic beam edge focusing electrodes, electrical connections, beam forming electrode structure, and beam collector electrode structure are all formed, as by printing, on the common face of a dielectric slab facing a sheet-shaped electron beam. A pair of slabs of homogeneous permanently magnetizable material is disposed overlaying the dielectric slab externally of the vacuum envelope. The magnet structure is permanently magnetized in a pattern of periodic permanent poles of alternating polarity taken in a direction along the beam path, whereby an extremely simplified tube structure is obtained.
In another feature of the present invention, a thermionic cathode is disposed intermediate the ends of two printed microwave interaction structures carried from the same face of a dielectric slab for projecting electron beams in opposite directions over the printed circuits to provide two microwave tubes within a common envelope.
In another feature of the present invention, plural microwave tubes are provided within a single envelope by printing a plurality of interaction circuits in side-byside relation on a common dielectric slab and projecting a stream of electrons over the printed circuits to obtain-a plurality of microwave tubes within a common envelope.
In another feature of the present invention, the permanently magnetized homogeneous slab of magnet material is flexible such that after a pattern of poles has been charged into the slab it is deformed, such as into a cylinder, to match the contour of the envelope of the beam to be focused.
Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an exploded schematic perspective view of a microwave tube incorporating features of the present invention,
FIG. 1A is an enlarged detail view of an alternative edge seal embodiment for a portion of the structure of FIG. 1 delineated by line lA-IA,
FIG. 2A is a perspective schematic view of a periodic magnetic focusing pole structure useful in the tube of FIG. 1,
FIG. 2B is a perspective plot of the axial magnetic field intensity for the structure of FIG. 2A,
FIG. 3A is a view similar to that of-FIG. 2 depicting an alternative embodiment of the magnetic focusing structure for the tube of FIG. 1, I
FIG. 3B is a perspective plot of the transverse magnetic field intensity for the structure of FIG. 3A,
FIG. 4 is a schematic cross-sectional view of a sheet electron beam depicting the space charge focusing forces,
FIG. Sis a plot of transverse and lateral electric defocusing field forces versus transverse extent of the electron beam of FIG. 4,
FIG. 6 is an enlarged cross-sectional view of a beam edge portion of the tube structure of FIG. 1 depicting the beam edge electrostatic focusing electrode structrue and its edge focusing electric field,
FIG. 7 is a plot of collector current in milliamps versus voltage on the electrostatic beam edge focusing electrode structure,-
FIG. 8 is a view similar to that of FIG. 1 depicting an alternative tube structure of the present invention, and
7 FIG. 9 is a schematic plan view of a combination oscillator and amplifier printed circuit tube incorporating features of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a printed circuit microwave beam tube 11 incorporating features of the present invention. A tube 11 includes a pair of ceramic slabs l2 and 13, as of alumina or beryllia ceramic, sealed together in a gas-tight manner in spaced apart relation via the intermediary of a suitable gastight spacing structure such as spacing ring 14 as of metal or ceramic. In an alternative embodiment of FIG. 1A, the ceramic slabs l2 and 13 are sealed at their peripheries, as by brazing, to metallic sealing frames 15 and 16 which in turn are sealed together as by heliarc welding at 20. The frames 15 and 16 replace spacer l4 and serve as a means for sealing the vacuum envelope of the tube.
A thermionic cathode emitter 17 is mounted to one of the ceramic plates 12 and 13, such as the lower plate 12, by soldering the heater leads of the thermionic cathode 17 to metalized leads 10 formed on the inner face of the lower ceramic plate 12. The thermionic cathode assembly 17 is preferably disposed at one end of the ceramic tube 11 in the space between the spaced ceramic plates 12 and 13 respectively.
A microwave interaction structure 18, such as a meanderline, an interdigital line, a double meanderline, or an array of distributed interaction klystron cavities or the like is formed on each of the opposed inner faces of the ceramic plates 12 and 13, respectively, and a beam collector electrode plate 119 is similarly formed on both of the lower and upper ceramic plates 12 and 13.
A pair of beam edge electrostatic beam focusing electrodes 21 are disposed at the side of each of the microwave interaction structures 18 on both the opposed inner surfaces of the lower and upper ceramic plates 12 and 13, respectively.
A pair of electron gun beam forming electrodes 20 are formed on the opposed inner surfaces of the lower and upper dielectric plates 12 and 13 between the upstream end of the microwave circuit 18 and the thermionic cathode emitter E7. The focus electrodes 21 are disposed in transverse registration across the beam path for initially transversely focusing the electron beam into the region between the printed slow wave circuits.
Electrical connections are made to each of the various electrode structures within the envelope by means of electrically conductive pins passing through the respective ceramic plates 12 and 13 and being connected to the respective electrodes by means of metalized pads and metalized holes bonded to the pins. The dielectric plates and spacer structures 12, I3 and 14 are all bonded together in a gas-tight manner to define a gastight envelope for the tube ill. The envelope is evacuated via a conventional pump-out tubulation, not shown, to a suitable low pressure, as of 10 torr.
A pair of sheet-shaped magnets 23 and 24 are disposed externally of the vacuum envelope abutting the outside faces of the lower and upper dielectric ceramic slabs l2 and 13, respectively. The sheet-shaped magnets 23 and 24 are made of a generally homogeneous permanently magnetizable magnetic material, such as ferromagnetic material, or permanently magnetizable particles embedded in a suitable binder such as silicon rubber. Suitable magnet materials include barium ferrite Ba Fm- 0 preferably oriented, and barium ferrite embedded in silicon rubber. Such materials preferably have a high coercive force, as of 5 2,000 Oresteds.
The sheet-shaped magnets 23 and 24 are permanently magnetized in a pattern generally indicated with respect to magnet 24, such pattern having a plurality of laterally directed magnetic poles of alternating polarity taken in a direction along the beam path as shown schematically in FIGS. 2 and 3. The permanent magnet pattern is readily charged into the slab-shaped magnet 24 by placing the magnetic sheet 24 over a suitable magnetizing fixture consisting of a generally comb-shaped magnetically permeable material, as of soft iron. The teeth of the comb have a vane shape generally conforming to the shape of the laterally directed poles to be charged into the magnetic sheet 24. Energizing coils are wound about the base of the individual vane-shaped teeth of the combshaped charging fixture with the direction of current through adjacent coils being in the opposite direction such as to generate, when energized with current, poles of alternating polarity taken in the direction down the spine of the comb-shaped charging fixture.
The magnetic fixture 24, which is to be charged, can be disposed between the mutually opposed teeth of two such comb-shaped charging fixtures with the transversely aligned opposed teeth of the combs being of opposite magnetic polarity such that the magnetic sheet 24 is charged uniformly through the sheet with poles of alternating polarity. Alternatively, the sheet 24 may be disposed over a single charging comb and charged. In this case, the magnetic sheet 24 is permanently magnetized with transverse directed poles opposite the ends of each of the vane-shaped teeth of the charging fixture and with longitudinal polarization in the outer regions of the fringing field between adjacent poles. An advantage to making the magnet structure 23 and 24 by charging the desired pole pattern is that, once a charging fixture has been made the desired magnet structure can be duplicated exactly without assembly and fabrication of individual magnets and spacers. Also, the intensity and period of the periodic magnet structure is readily varied down the length of the magnet structure by varying the ampere turns around the individual vane-shaped teeth of the charging fixture and by varying the spacing between adjacent teeth of the charging fixture.
The permanently magnetized sheet magnets 23 and 24, with their respective pole patterns, are disposed on opposite sides of the ceramic sheets 12 and 13 with their magnetic poles in transverse registration. in one type of magnetic focusing, as shown in FIG. 2, the opposed poles in the opposed sheets 23 and 24 have the same magnetic polarity to produce periodic magnetic focusing of the sheet-shaped beam in the manner as disclosed in U. S. Pat. No. 3,102,211. As an alternative, the pole patterns in the opposed magnetic sheets 23 and 24 may be arranged such that opposed type magnetic poles are disposed in transverse registration as shown in FIG. 3. This type of magnetic deflection focusing is disclosed and claimed in U. S. Pat. No. 3,013,173.
In a typical example of a watt, L-band, CW microwave amplifier tube 11, the slow wave circuit 18 has a lateral width of approximately 0.80 inches and an axial length of approximately 5 inches. The electron beam has a lateral width of approximately 0.700 inches and a thickness of approximately 0.055 inches. Slow wave circuits 18 are formed as by printed circuit techniques on the inner major faces of the opposed ceramic sheets 12 and 13, each slab having a thickness of approximately 0.100 inches and being spaced apart from the opposed slab by approximately 0.120 inches. The electrostatic beam edge focusing electrodes 21 are spaced by approximately 0.060 inches from the adjacent edge of the slow wave circuit 18 and each electrode 21 has a lateral width of approximately 0.050 inches. The sheet-shaped magnets 23 and 24 each have a lateral width of approximately 1.00 inches and a thickness of approximately 0.100 inches and extend for substantially the entire axial length of the tube. In this case, the magnet structure had a period of 0.5 inch from one north pole to the succeeding north pole taken along the beam path. The periodic beam focusing field had a peak longitudinal component of magnetic field intensity in the midplane of the beam of 200 gauss.
In the case where the sheet-shaped magnets 23 and 24 are magnetized straight through, such that the material between adjacent poles is not permanently magnetized, a magnetically permeable member, such as a sheet of soft iron, not shown, is preferably disposed over the outer major face of the sheet-shaped magnets 23 and 24 to serve as a return magnetic flux path of high magnetic permeability.
Referring now to FIGS. 4-6 the effect of the beam edge electrostatic focusing electrodes 21 is shown. In FIG. 4, the sheet-shaped electron beam is shown at 27 with electrostatic space charge defocusing forces as indicated by the arrows radiating away from the beam 27. From the direction of the arrows, it is seen that the defocusing forces are substantially transverse in the midlateral sections of the beam 27 but, near the beam edges, the defocusing forces become substantially lateral.
Referring now to FIG. 5 there is shown the relative amplitude of the transverse and lateral spacecharged defocusing electric fields. From the curve it is shown that the transverse defocusing electric field falls off as the lateral defocusing field increases.
Referring now to FIG. 6 there is shown the focusing electric field lines produced by the beam edge electrostatic focusing electrodes 21. The beam 27 will have a certain beam voltage, as of +900 volts relative to the cathode potential. Typically, the beam voltage is at ground potential which is also the potential on the slow wave circuit 18 and the cathode 17 is run at a negative potential. The difference between the slow wave circuit potential and the cathode potential corresponds to the beam voltage.
The beam edge focusing electrodes 21 are operated at a potential negative with respect to the beam potential and potential of the circuit 18. The electrostatic force on the electrons is indicated by arrows 28 and it is seen that these arrows, in the mid-transverse plane of the beam at the edge of the beam, have a maximum amplitude tending to force the electrons back toward the mid-lateral plane of the beam 27.
It turns out that the potential on the edge focusing electrodes 21 relative to the beam potential and the potential on the circuit 18 is not critical. This is shown in FIG. 7 wherein collector currents in milliamps is plotted versus voltage on the edge focus electrodes 21. From the curve it is seen that once a minimum voltage has been established between the electrostatic focus electrode and the beam voltage and circuit potential, as of 50 volts, that a further increase of the voltage difference has very little effect on beam transmission. It is also seen that a relatively high beam transmission efficiency is obtained, as of 95 percent.
In a typical example of a traveling wave tube of the type shown in Flg. 1, beam transmission of 92 percent was obtainable. With full RF drive and 50 percent collector depression the beam transmission fell only to percent. The instantaneous bandwidth of the tube was greater than 20 percent centered at an L-band frequency of approximately 1.1 GHz while providing 15 watts of CW output power. With these results at L- band, it is quite feasible to obtain approximately 2 kilowatts pulse power in the frequency range of 3.1 to 3.5 GHz. The manufacturing cost of the printed circuit tube of FIG. 1 is approximately one-tenth of the cost of a conventional traveling wave tube to obtain the same output performance.
Referring now to FIG. 8, there is shown an alternative microwave tube embodiment 30 of the present invention. In FIG. 8, the tube 30 is substantially the same as that previously described with regard to FIG. 1 with the exception that the longitudinal and lateral extent of the ceramic plates 12 and 13 has been extended to accommodate a plurality of parallel microwave interaction circuits l8 and formed on both of the mutually opposed faces of the ceramic plates 12 and 13, respectively. In addition, the thermionic cathode emitter assembly 17 has been moved to the center of the tube and the collector electrodes 19 are disposed at opposite ends of the ceramic slabs 12 and 13 such that the electron beams are directed from both sides of the thermionic cathode emitter 17 toward opposite ends of the individual microwave interaction circuits 18 to the collector assemblies 19 at opposite ends of the tube. In this manner, a single cathode emitter 17 can serve to provide an electron beam for a plurality of individual tubes connected in parallel. In a particular example, as depicted in Flg. 8, there are 10 tubes connected in parallel. The microwave circuits and the connections for the individual tubes are readily formed by printed circuit techniques such that the fabrication cost for the 10 tubes is substantially the same as it would be for one tube. The'RF outputs of the individual circuits 18 are taken out through suitable RF output connectors 3]. arrayed at opposite ends of the composite tube 30. The individual RF outputs may be used individually or connected in parallel for increasing the power output capability of the tube 30. The thermionic cathode 17 may comprise, for example, a directly heated thoriated tungsten ribbon.
Referring now to FIG. 9, there is shown, in plan view, an alternative microwave tube embodiment 35 of the present invention. In tube 35 of FIG. 9 the microwave circuit 18 includes two circuit portions, circuit portion 18 is a forward wave amplifier circuit, and circuit portion 18 is an interdigital backward wave oscillator circuit. The output of the backward wave oscillator circuit 18 is fed into the input of the forward wave slow wave circuit via printed circuit line 33 disposed at the upstream end of the electron beam. The beam is emitted by a thermionic emitter 17 such that the sheet beam is common to both slow wave circuits l8 and 18' and is collected by a common collector electrode 19.
mogeneous permanently magnetizable member disposed on one side of said sheet-shaped beam and said Although, thus far in the above description, the magl nets 23 and 24 have been described as they would be employed for focusing a sheet beam it is to be understood that the technique is also applicable to focusing of solid and hollow cylindrical vbeams. In the case of a solid cylindrical beam, the magnet may comprise a hollow cylinder which is charged with a pattern of axially spaced ring-shaped pole regions of alternating magnetic polarity taken in the axial direction along the beam path.
The hollow cylindrical magnet may be formed by charging the pattern into a magnetic cylinder or by charging the pattern into a sheet or slab of flexible magnetic material, such as silicon rubber sheet or slab having a homogeneous suspension of permanently magnetizable magnetic particles embedded therein, and then bending the magnetized sheet into a cylinder to form the axially spaced pole pattern of ring-shaped poles of alternating polarity.
In the case of a hollow cylindrical beam, two such concentrically disposed cylindrical magnets may" be employed for focusing the annular beam passable coaxially of and between the pair of cylindrical permanently magnetized magnet structures.
What is claimed is:
1. In a magnetically focused beam tube, an evacuable envelope structure, means within said envelope for forming and projecting a beam of charged particles over a predetermined beam path, electrical circuit means disposed along the beam path in electromagnetic wave energy exchanging relation with the beam of charged particles, periodic magnetic focus means disposed along the beam path for producing a periodic magnetic field within the beam path for focusing the beam along a predetermined beam path, said periodic magnetic focus means including, a generally homogeneous structure consisting of permanently magnetizable material, said magnetizable structure being permanently magnetized in a pattern of periodic permanent poles of alternating polarity taken in a direction along the beam path.
2. The apparatus of claim 1 wherein said beam forming means forms a sheet-shaped beam, and wherein said homogeneous structure includes, a generally homagnetizable member being permanently magnetized in a pattern having periodic permanent poles of alternating polarity taken in a direction along the beam path.
3. The apparatus of claim ll wherein the period of said pattern of permanent magnetic poles changes in a direction taken along the beam path.
4. The apparatus of claim 1 wherein said magnetizable structure has an even surface facing said beam.
5. The apparatus of claim 2 wherein said magnetizable structure comprises a slab of substantially homogeneous permanently magnetizable material having a planar face facing said beam.
6. The apparatus of claim 5 wherein said magnet slab is a slab of ferrite magnet material.
7. The apparatus of claim 5 wherein said magnet slab is a slab of permanently magnetizable magnet particles embedded in a binder material.
8. The apparatus of claim 7 wherein said binder material is flexible such that said slab is flexible.
9. The apparatus of claim 2 including, a dielectric slab having major and minor faces with a major face disposed facing a major face of said sheet-shaped beam, and wherein said magnetizable member includes a slab of substantially homogeneous permanently magnetizable material disposed overlaying said dielectric slab, and said magnetic slab member having major and minor faces, and a major face of said magnetic member being disposed facing a major face of said dielectric slab.
10. The apparatus of claim 9 wherein said permanently magnetizable structure includes a pair of said magnetizable slabs disposed on opposite sides of said sheet-shaped beam with their respective major faces disposed facing a corresponding major face of said sheet-shaped beam. I
11. The apparatus of claim 9 wherein said electrical circuit is a microwave interaction structure formed on said major face of said dielectric slab which faces the corresponding major face of said sheet-shaped beam.
12. The apparatus of claim 11 including, electrostatic edge focusing electrode means disposed adjacent the edge regions of said slow wave circuit in electrical insulative relation thereto and extending along the opposte edges of said sheet-shaped beam for constraining lateral expansion of said sheet-shaped beam.
13. The apparatus of claim 12 wherein said edge focusing electrode means is disposed on said major face of said dielectric slab which faces said major face of said beam.
14. The apparatus of claim 13 wherein the beam of charged particles is a beam of electrons, and wherein said beam forming and projecting means includes a thermionic cathode emitter means disposed at the upstream end of said beam path for generating the beam of electrons, current carrying a lead means for supplying heater current to said thermionic cathode emitter, and wherein said dielectric slab has an even major face facing said corresponding major face of said electron beam, and wherein said slow wave circuit, edge focusing electrode means, and said heater lead means are all formed directly on and lie substantially entirely on said even major face of said dielectric slab which faces saidsheet-shaped electron beam.
15. The apparatus of claim 14 wherein said even major face of said dielectric slab is planar.
16. The apparatus of claim 15 including beam collector electrode means disposed on and lying substantially entirely on said planar major face of said dielectric slab at the terminal end of said beam path.
17. The apparatus of claim 16 including, beam focus electrode structure disposed on and lying substantially entirely on said planar major face of said dielectirc slab at the upstream end of said beam path intermediate said cathode emitter means and said slow wave circuit means.
18. The apparatus of claim 11 wherein said electrical circuit means includes a pair of elongated microwave interaction structure means disposed of said major face of said dielectirc slab in end-to-end relation, and wherein said beam forming and projecting means includes a thermionic cathode emitter disposed intermediate the adjacent ends of said pair of microwave interaction structure means for projecting a pair of electron beams in opposite direction along said pair of microwave interaction structure means.
19. The apparatus of claim 11 wherein said electrical circuit means includes a plurality of elongated microwave interaction structure means disposed on and lying substantially entirely on said major face of said dielectric slab in side-by-side relation.